In a coaxial cable which is used for an electric wire for communication, a foamed coaxial cable formed by constituting a dielectric, provided around a center conductor, by a foamed body has a small dielectric constant of the dielectric and enables high-speed transmission of a signal, and is thus used as a signal transmission cable. For example, in an example of the related art disclosed in Patent Document 1, a foamed insulating cable is used as an electric wire for communication. On the other hand, in a coaxial cable used for a medical instrument and the like, there have recently been demands for a reduction in the diameter of the coaxial cable, with reductions in size and weight and power saving of the medical instrument and the like. For example, the foamed coaxial cable has been used for a transmission cable, such as a medical ultrasonic probe cable and an endoscope cable, which has a large number of conductors, and there have been particularly demands for a reduction in the diameter and flexibility of the cable from the viewpoint of operability.